I. Field of the Invention
The present invention relates to an air-fuel ratio sensor and an air-fuel ratio detecting apparatus using the same.
II. Description of the Prior Art
Various conventional air-fuel ratio sensors have been proposed. In a conventional air-fuel ratio sensor (Japanese Laid-Open Patent Application No. 58-61455), a sensor for detecting oxygen gas in an exhaust gas from a vehicle or the like, a resistive type oxide semiconductor sensor whose resistance changes abruptly at a theoretical air-fuel ratio (to be referred to as .lambda.=1 hereinafter), and a limiting electric current type oxygen sensor whose current changes in accordance with ambient oxygen concentration upon application of a constant voltage thereto are electrically connected in parallel with each other, and the resulting sensor is heated by a heater arranged near the sensor. In another conventional air-fuel ratio sensor (Japanese Laid-Open Patent Application No. 58-30654), the above three component sensors are formed adjacent to each other on a substrate with a heater.
Although these air-fuel ratio sensors have a compact structure and high response with an air-fuel ratio detection range unmatched by other conventional sensors, there is still room for improvement. In general, exhaust gas temperatures in the engine of a vehicle or the like vary with different operation states, so a wide operating temperature range from a low temperature to a high temperature is required for an air-fuel ratio sensor. In addition to accurate detection of the air-fuel ratio, low power consumption is also necessary.
No conventional air-fuel sensors, however, can meet these requirements.
In order to obtain a wide measurement range by combining two types of sensors, their respective processing circuits must be integrated to obtain a single air-fuel ratio output, since two independent processing circuits limit the compactness of the system configuration. According to extensive studies by the present inventors, such integration entailed a problem to be solved.
A large current flows in the limiting electric current type oxygen sensor portion in regions where the air-fuel ratio .lambda. is smaller than 1, irrespective of the value of .lambda., thus establishing a two-valued function.
In order to solve the above problem, the limiting electric current type oxygen sensor portion is set to switch off when the air-fuel ratio .lambda. is less than 1. However, simple switching causes pulse noise, thus presenting yet another problem.